| As a long-term and complicated ecological project, converting farmland into forest or grassland will locally influence the social and economic development as well as improve the ecological environment conditions. A feasible follow-up compensatory mechanism has a great importance with the project's sustainable development. Karst peak-cluster-depression region is mainly located in the south slope of karst area in Southwest China, with the problems of fragile ecological environment, sharp conflicts between local and farmland, intensive human disturbance, and severe rocky desertification, which threatened the ecological safety of Yangtse River and Zhujiang River. Therefore, ecological recovery and restoration are urgently needed in karst peak-cluster-depression region. Given this situation, the project of converting farmland into forest or grassland was carried into execution in 2002 in the region. Now the problems that need to be crucially resolved, including evaluation the ecological effects and the coupling conditions of vegetation and soil effects.Taken a landscape unit of conversion of cropland to forest or grassland in karst peak-cluster-depression (i.e. the Guzhou Ecological Recovery and Restoration Demonstration Section of Karst Peak-cluster-depression, Institute of Subtropical Agriculture, CAS, Huanjiang, Guangxi) as the studied object and slope farmland as contrast, the paper started with the analysis of the ecological effects of vegetation and soil, and then probed into the reciprocity mechanism of vegetation and soil based on typical correlation analysis, and then demonstrated the main factors of the conversion ecosystems of cropland to forest and grassland, and then established the coupling model and harmonized coupling model of vegetation and soil, at last, by the cluster analysis, the paper screened out the optimized conversion models. The main results of the study could be concluded as follows:(1) Two-way indicator species analysis (TWINSPAN) had divided the vegetation into 8 community types in the converting areas at the fourth level, combined with the situation and ecological meaning. There were 65 woody species belonging to 26 families, with low diversity and construction index but great variation, and with complex life forms. The soil quality was better than the red soil at the same latitude, but much worse than the limestone soil covered with vegetation at the same latitude. The ecosystem of converting areas was in the primary stage of natural recovery with weak vegetation stability and soil low-level spare status.(2) Vegetation effects differed in different conversion models. The biomass was small due to only 5 years, in the order of Zenia insignis+Guimu No.1 Forage, Zenia insignis, Toona sinensis, Castanea mollissima, Guimu No.1 Forage, leaving land uncultivated, and Citrus reticulate models. The biomass mainly distributed on the aimed trees and grass, while firewood forest mainly on stocks and economic forest mainly on stocks and branches. The species abundance, Shannon index, and evenness index were low in all the converting models, which suggested the models were in the higher level with less species. The content, storage, and distribution pattern of nutrient elements had different details in the converting models. The storage distribution of organic carbon and potassium in the organs was mainly affected by the corresponding biomass, while that of nitrogen and phosphorus by both the corresponding biomass and content, and that of mineral nutrients by the corresponding content. The storage of main nutrients was in the order of carbon, nitrogen, potassium, and phosphorus. Meanwhile, the content of mineral nutrients was much higher than in Pinus massomiana plantation and Pinus elliottii plantation in the same latitude, in the order of SiO2, CaO, Al2O3, Fe2O3, MgO, and MnO. Given the vegetation integrative effects, the seven converting models could be divided into 5 classes, the models of Zenia insignis+Guimu No.1 Forage and Guimu No.1 Forage as the first class, the model of Citrus reticulate as the second class, the model of leaving land uncultivated as the third class, model of Zenia insignis as the fourth class, the models of Toona sinensis and Castanea mollissima as the fifth class.(3) There was local superiority of high content of soil water in the drought karst condition in karst peak-cluster-depression region. Taken slope farmland as contrast, all the models had some degrees to an increase soil water content, porosity, organic matter, nitrogen, phosphorus, potassium, and soil microbial biomass, the amount of soil microbial populations, the ratio of microbial carbon biomass (BC) to microbial nitrogen biomass (BN), and the content of SiO2, but a decrease in other soil mineral nutrient content. This suggested that the converting models had favorable effects on soil, and those could be divided into 4 types of soil remarkable improvement, soil improvement, soil slow improvement, and the soil lagging improvement. There were ten soil main key factors, and the relationships between soil physicochemical properties, mineral properties, and soil microbial were closely.(4) For nutrients in vegetation, the amount of absorption, retention, and return, as well as for nutrient elements, the amount of absorption, retention, and return, use coefficient, recycling coefficient, and turnover time differed in details in the seven models. The whole trend performed as great use coefficient, low recycling coefficient, and close turnover time. Mineral nutrients had greater use coefficient and recycling coefficient, fewer turn over time, and faster biological cycle than nitrogen, carbon, and potassium. This indicated that there were similar recycle characteristics of biogeography chemical cycle in the models and the elements, respectively.(5) There were remarkable and significant remarkable canonical correlations between vegetation and soil physicochemical properties, between vegetation and soil microbial properties, between soil physicochemical properties and soil microbial properties, between mineral nutrients in aboveground and belowground, based on the canonical correlation analysis. Soil available nutrients on vegetation growth and diversity, the amount of microbial population on vegetation growth, the amount of microbial population on soil available nitrogen and phosphorus, silicon in soil on calcium in vegetation, had greatest effects, respectively.(6) Both the coupling models and harmonized coupling models differed in the converting models in karst peak-cluster-depression region. The coupling situation was still not good due to the short converting time. Cluster analysis divided the converting models into 4 classes, Toona sinensis and Castanea mollissima models as first class, Citrus reticulate and Zenia insignis+Guimu No.1 Forage models as the second class, Zenia insignis and Guimu No.1 Forage models as third class, the model of leaving land uncultivated as the fourth class. There were great variations among the classes and similar ecological effects within the class.(8) On a small scale, the paper firstly analyzed the vegetation effects and soil effects, as well as the reciprocity mechanism between them, and then found the key factors of the ecosystems, and then screened out the optimized converting models, and last constructed vegetation and soil coupling models and harmonized coupling models of converting farmland to forest or grassland in karst cluster-peak-depression region, Southwest of China. This would provide theoretical and practice guide to evaluation of the ecological service function, construction of ecological and economic coupling model and harmonized coupling model, whole implementation of the converting project, and a nice ecological and economic development in the area.
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